They are mounted on either side of the Gimbal Ring.
One sender is for the Trim Gauge and the other sender is for the Trim Limit.

The Trim Limit Switch is mounted on the port side and the Trim Position Switch is mounted on the starboard side.

The switches are prone to damage from water intrusion. Water enters the switch and ruins the contacts. The wires tend to soften and/or rot with age and
water leaks into the switch through the wires.

This is why the new switches are sold with the new wires are already attached at the factory.
Always replace the wires when replacing the switches.

Symptoms
The Trim Position Switch operates the Trim Gauge.
When the Trim Position Switch fails, the gauge usually pegs to "full up".

The Trim Limit Switch prevents the driver from 'over-trimming" the outdrive while under way.
It is an important limiting feature which prevents damage to the drive and/or loss of control of the vessel.
Trimming the drive unit up too high while the boat is running can cause serious damage and/or injury.
When this switch fails, the TRIM BUTTON does not work. Only the TRAILER BUTTON will work.

When one switch fails, you will end up replacing both switches. This is because the switches are sold in sets of two.
Both the Trim Limit and the Trim Sender switches are sold together as a pair.

Replacing the switches requires removal of the drive and bell housing.
These are the same procedures for replacing the Ujoint Bellows.

Trim Sender Units
The trim senders are no longer sold individually.
The Port switch is the Trim Limit Switch. This limits how far up the drive will trim.
The Starboard switch is the Trim Level Sender switch. This sends a signal to the trim gauge.

If one or the other malfunctions, you will need to get the entire Kit.
The Kits comes with both senders. The senders are pre-wired from the factory
with new wires.
Mercruiser did this because bad wires were the leading cause for switch failure.

805320A03 Single Station Trim Sender Kit $92.50

If you have Dual Stations (for example, a gauge below and a gauge on the bridge),
You will also need to purchase the Dual Station sender from below.